1. Field of the Invention
The present invention relates to a rotary encoder that detects a rotation angle.
2. Description of the Related Art
Previously, a rotary encoder has been used for measuring an angle in a machine tool, an FA apparatus, or the like. An angle detecting method of the rotary encoder is roughly divided into an incremental method and an absolute method. In the incremental method as disclosed in Japanese Patent Laid-Open No. 2006-214929, the configuration is simple, but there are disadvantages that angle information is lost when the power is turned off and an error is accumulated by an external noise. On the other hand, the rotary encoder using the absolute method commonly has advantages that it does not accumulate errors and is highly accurate and that it is unnecessary to move to a home position even when the power is turned off.
The rotary encoder using the absolute method that detects an absolute position is, for example, disclosed in Japanese Patent Laid-Open No. H02-168115. According to the rotary encoder disclosed in Japanese Patent Laid-Open No. H02-168115, a phase synchronization of an upper-level (absolute) pattern and a lower-level (increment) pattern is performed to be able to detect a highly-accurate absolute signal.
Commonly, it is difficult to align a pattern center with a rotation center of the rotary scale in the rotary encoder, and an error of the detection angle is generated by the eccentricity generated by a mechanical error. In order to suppress the error caused by this eccentricity, Japanese Patent Publication No. H07-069190 discloses a rotary encoder which corrects a displacement signal detected by a detector for detecting the displacement using an eccentric error obtained by a detector for detecting the eccentric error. Japanese Patent Laid-Open No. H11-108698 discloses a rotary encoder which corrects an eccentricity using two detecting parts disposed at positions different by 180 degrees from each other with respect to one pattern.
However, in the rotary encoder which detects the absolute position using the upper-level detection and the lower-level detection, when a detector and a pattern that are used for detecting the eccentric error are further provided, the size of the entire scale and encoder is enlarged and the cost is increased. Even when two detecting parts are used for each of the upper-level detection and the lower-level detection, the size of the entire encoder is enlarged and the cost is increased.